A multimode metal oxide-based gas sensor to enhance selectivity towards volatile organic compounds

Abstract

This thesis presents the design, manufacturing, and characterisation of a multimode metal oxide-based gas sensor. The aim of the multimode sensor is to enhance selectivity by integrating a chemiresistive sensor with a quartz crystal microbalance (QCM) into a single microscale device. The motivation for this work is the limited selectivity of low-cost gas sensors compared to laboratory systems, while the proposed design maintains the ambition of low-cost fabrication, small size, and industrial applicability. The proposed sensor combines interdigitated chemiresistive electrodes, a quartz resonator, platinum heaters, temperature-sensing functionality, and a metal-oxide-sensitive layer into a single device. The design is supported by analytical calculations and thermal simulations, after which the sensor is manufactured using standard MEMS-compatible processes, including lithography, lift-off metallisation, TEOS deposition, metal oxide printing, and packaging.

The fabricated sensor is characterised using a platinum-doped SnO2 sensing layer and a sequential electrical read-out of metal-oxide resistance and resonance frequency. The chemiresistive mode shows the strongest performance, including VOC-1 detection and an estimated limit of detection. The QCM sensor shows a resonance frequency of approximately 7.67 MHz at 50 °C and a Q-factor of about 550, but its performance is limited by low resonance quality and a relatively high detection limit. Nevertheless, combining both sensing principles yields gas-dependent relations between chemiresistive sensitivity and frequency response for VOC-1, VOC-2, and VOC-3. This demonstrates that the monolithic multimode concept provides additional discriminatory information compared with either sensing principle individually. It is concluded that monolithic integration of a chemiresistive and QCM sensor can enhance selectivity, but further optimisation of the resonator design, measurement electronics, and thermal control is required before the concept can be used as a robust, selective detector at low concentration levels.